The present invention relates to a belt-type continuously variable transmission comprising a continuously variable transmission mechanism in which a belt passes over a drive pulley and a driven pulley, a frictional engagement element that interrupts drive from the engine, and means that control the side-pressure that is supplied to the cylinder chambers of the two pulleys.
A belt-type continuously variable transmission of this type is arranged such that the drive from an engine is transmitted through a continuously variable transmission mechanism comprising a drive pulley, driven pulley and a belt that passes over these two pulleys, and a frictional engagement element that interrupts drive of the engine; such a belt-type continuously variable transmission is employed in vehicles etc. In such a belt-type continuously variable transmission, if the transmission torque of the belt exceeds an allowed value (the torque such that, if the transmission torque of the belt rises above this, the belt slips), the belt slips, which is undesirable from the point of view of both friction and costs. In order to prevent this situation, the technique of setting a maximum torque lower than the aforementioned allowed value by means of a frictional engagement element has become publicly known by Laid-open Japanese Patent publication number H. 4-228960 etc; with this technique, if the transmission torque of the belt rises, the frictional engagement element starts to slip before the torque exceeds the aforementioned allowed value, so there is no possibility of the transmission torque of the belt exceeding this allowed value. It is desirable that the transmission torque of this frictional engagement element should be a value that is as close as possible to the aforementioned allowed value, but, since the allowed value constantly changes in response to the pulley force (side-pressure) with which the belt is gripped, usually the transmission torque of the frictional engagement element is electronically controlled by means of a linear solenoid valve etc.
However, in such a construction in which transmission torque control of the frictional engagement element is performed using a linear solenoid valve, the linear solenoid valve is expensive, giving rise to the problem of high manufacturing costs.
An object of the present invention is to provide a belt-type continuously variable transmission of a construction that is capable of preventing belt slippage by means of a straightforward construction that does not employ a linear solenoid valve.
A belt-type continuously variable transmission according to the present invention comprises: a continuously variable transmission mechanism wherein a belt passes over a drive pulley connected with an input shaft (for example input shaft 20 in the embodiments) that inputs drive from a motor (for example, an engine in the embodiments) and a driven pulley connected with an output shaft (for example, left and right axle shafts S4, S5 in the embodiments) and wherein gear-change action is performed by receiving supply of working hydraulic fluid to a drive cylinder for pulley width setting of the drive pulley and a driven cylinder for pulley width setting of the driven pulley; a frictional engagement element provided between the motor and the drive pulley or between the driven pulley and the output shaft and that performs transmission of the drive from the motor with an engagement capacity responsive to the working hydraulic fluid pressure that is supplied or that is capable of cutting off transmission of drive by release thereof and side-pressure supply means (for example, in the embodiment, hydraulic pump 100, regulator valve 110, first side-pressure control linear solenoid valve 120 and second side-pressure control linear solenoid valve 124) that supply a first side-pressure that determines the transmission torque capacity of the continuously variable transmission mechanism to one of the two cylinders and that supply a second side-pressure whereby gear-change action of the continuously variable transmission mechanism is performed to the other of the two cylinders; the first side-pressure being supplied to the frictional engagement element as the working hydraulic fluid pressure.
In a belt-type continuously variable transmission mechanism according to the present invention, the first side-pressure that determines the transmission capacity of the continuously variable transmission mechanism i.e. the torque that can be transmitted between the two pulleys without producing belt slippage is employed as the working hydraulic fluid pressure of the frictional engagement element, and the transmission torque capacity of the frictional engagement element is varied together with the transmission torque capacity of the continuously variable transmission mechanism. Consequently, if the transmission torque capacity of the frictional engagement element is set as required (for example, the first side-pressure is supplied to the frictional engagement element after being reduced in pressure, or the number of frictional plates constituting the frictional engagement element and/or their coefficient of friction are adjusted) so as to be somewhat lower than the transmission torque capacity of the continuously variable transmission mechanism, even if a torque exceeding the transmission torque capacity of the continuously variable transmission mechanism is input to the belt-type continuously variable transmission, slippage of the frictional engagement element can be induced before occurrence of belt slippage in the continuously variable transmission mechanism, thereby enabling damage to the belt due to slippage to be prevented. Also, with this construction, engagement capacity control of the frictional engagement element is performed without requiring an expensive linear solenoid valve, so manufacturing costs can be considerably reduced compared with the case where a linear solenoid valve is employed, and in regard to reliability with respect to belt slippage it is superior to a construction using a linear solenoid valve and is also of improved safety.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.